The atmosphere in which physicochemical transformations are conducted is of paramount importance in many industrial operations. In particular, water vapor measured in amounts as low as a few parts per million by volume assumes important measurement focus. The presence of moisture in unwanted proportions can cause retardation of reaction. Oxidation, inhibited polymeric reaction, catastrophic freeze-ups corrosion, and impairment of catalyst activity. Moisture measurement is thus a critical need in many industries including papermaking, petroleum refining, paint manufacturing, heat treating, food production, glass manufacture, and chemical making.
In many industries the moisture content of the raw materials and products are also critical. The moisture content of materials can affect their physical properties, including mechanical strength and electrical properties, their physical appearance and their ability to be processed.
For instance, in the polymer processing industry both the processability of the polymer and the physical properties of the end product depend upon the moisture content of the granules used as raw material. The moisture content of the final product can, in some instances, markedly affect the mechanical properties such as impact strength.
Moisture content of polyolefins is an important parameter for polymer manufacturers and fabricators because water can affect the physical and appearance properties of the processed product. For example, exposure to water has been shown to cause a decrease in tensile strength of polyethylene. Similarly, since polyolefins are molded or extruded at temperatures above the boiling point of water, excessive amounts of moisture present in the raw resin can cause bubbles and streaks in the finished product. In high voltage electrical cables, polyolefin insulation materials can prematurely fail due to the presence of moisture. The moisture can lead to the development of "water trees" or low resistance leakage paths producing failure of the cable under actual use conditions.
Certan grades of thermoplastic polyester resin should be dried before injection molding in order to maintain their optimum properties. Some polyesters should not be molded unless they are adequately dried. Of course other polymers also can be benefited by drying. This is especially true when melt temperatures exceed recommended limits or when extended mold cycle times are expected. In some instances moisture levels as low as 0.02% are recommended for optimum processing.
Drying of polymers is frequently determined by the length of time that the resin is dried at a given temperature. Such drying periods can vary from a few hours to days. However, a quantitative check of the moisture level of the resin is often desirable. For such a checking technique to be useful in a mold shop it should be quick, simple and utilize inexpensive equipment.
One technique for simple moisture level determination is called the test tube/hot block technique (TTHB). It is based on the fact that moisture present in resin will vaporize when melted in a closed test tube. This moisture will condense, as it cools, in the form of tiny water droplets on the side of the glass tube. The surface area covered by this condensation on the tube can then be correlated to the moisture content of the pre-heated resin. The amount of surface area on the test tube wall covered by condensation corresponds directly to the moisture level of the resin.
Another technique is called T.V.I. after the engineer who developed it. In brief, this method entails heating a few pellets of polymer to their melting point and observing whether bubbles are present, indicating moisture in the resin, or absent, indicating a dry material.
However, both of these simple methods are primarily qualitative or semi-quantitation at best. It is readily seen that both are subject to many variables and open to considerable error if accurate moisture data is necessary.
The traditional technique for accurate analysis of moisture in polymer (ASTM Method D-789) involves a vacuum distillation followed by Karl-Fischer titration of the moisture. This method suffers from the disadvantages of being time consuming, costly in reagents and requiring a delicate laboratory technique.
A widely used method for accurate analysis of moisture in polymers is called moisture evolution analysis. The sample is heated to an operator controlled temperature in an oven to drive off any water. Moisture from the heated sample is picked up by a continuously flowing stream of externally dried nitrogen and is carried into an electrolytic cell to determine moisture content. Although relatively simple, this method still requires accurate weighing of the sample and must be conducted off line.
A method has been described for determining the moisture content of plastics, molding powders, fillers, etc. from the amplitude of the nuclear magnetic resonance (NMR) signal. Industrial NMR spectrometers for determination of moisture content have been described. However the method is still an off-line procedure and accuracy at low moisture levels of 1 to 10% may be no better than 0.5%.
The free (unbound) moisture content of many dielectric materials can be accurately measured with microwave techniques. Microwaves are strongly absorbed and scattered by water molecules because water exhibits a broadband rotational relaxation in the microwave region. Because many completely dry host materials are quite transparent in the same frequency range, a moisture-measuring technique is possible. This technique has found wide use on both a continuous process and laboratory sample basis, especially for plastic and ceramic materials. The main disadvantage of using microwave techniques for measuring moisture content is that they are not very sensitive at low moisture levels, particularly at moisture contents less than 1% where a substantial percent of the water molecules in the polymer may not be free.
Thus, it can be seen that a fast, accurate method for measuring the moisture content of dielectric materials such as polymers, particularly at low moisture levels is desired. Such a method preferably should be suitable for on-line continuous monitoring of moisture levels and capable of accepting large sample sizes.
Dielectric properties and the electrical conductivity of polymers have been widely studied by a large number of investigators. These studies were undertaken for a variety of reasons such as studying the molecular structure of a material, studying the mechanics of conduction in a material, etc. In conducting these studies it was appreciated that humidity and moisture in the material would affect the electrical properties and efforts were made to insure dryness for the studies. Several investigators have reported the effects of moisture on the electrical properties of polymers. For instance, in September 1956, Rushton and Russell, published a technical report on "The Dielectric Properties on Nylon", ERA Technology Ltd. report L/T355. In the report Rushton and Russell illustrate the variation of permittivity (E) and loss tangent (tan .delta.) with frequency for nylon having low moisture content and for dried nylon. However, neither Rushton and Russell nor other investigators suggested that the measurement of dielectric properties of a material at low frequencies would be useful for determining the moisture content of such materials.